Salmonella enterica causes significant disease in humans and animals. Like many gram-negative pathogens utilizes its flagellar organelles for motility. Flagellar motility is essential for Salmonella virulence and the establishment of biofilms. Assembly of the flagellum requires a specialized secretion apparatus, which is a member of family of secretion systems, termed type III secretion (T3S) systems that are used by plant and animal pathogens to secrete virulence determinants into host cells. The flagellar T3S system exports proteins from the cytoplasm into a central channel of the flagellar structure during the assembly process. Proteins polymerize into place at the tip of the elongating structure. This proposal will support research designed to characterize the flagellar assembly process and determine the mechanisms used by the flagellar T3S system that selects specific substrates and what determines how substrates are ordered for secretion with respect to each other. This proposal specifically addresses the mechanisms of secretion substrate selection, the type III secretion-specificity switch mediated by FliK, the flagellar rod assembly, and the formation of the PL-ring outer membrane pore that couples rod completion to hook growth initiation. In addition, the coupling of flagellar gene expression to the global regulatory pathways at the flagellar master control operon, flhDC, will be analyzed. In addition the mechanisms by which the numerous regulators we recently discovered control flhDC both at the level of operon transcription and post-transcriptional control. Regulators of flagellar gene expression include H- NS, CrsA, PhoPQ, DnaK, cAMP-CAP, LhrA, RtsB, SlyA FimZ, PefI-SrgD, RcsB, OmpR, EcnR, FliT and YdiV. Research will focus on the coupling of flhDC expression with SpiI virulence gene expression through the recently discovered FliZ-HilD-EcnR pathway, and the putative cyclic di-GMP phosphodiesterase protein YdiV.